Most likely some pictures have been taken, but not necessarily that many. Perhaps someone just inserted a memory card that happened to contain IMG_3879.JPG and took one test shot – usually, it will be named IMG_3880.JPG even if the camera is brand-new. Now if you insert an empty memory card, the camera will remember the previous file name and your first photo will be named IMG_3881.JPG.
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Jukka SuomelaApr 2 '11 at 22:08

thank you for your answer: that was my idea too, but what about batteries shuttercount? Is it resetted when I reload the Battery?
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HaiperlinkApr 2 '11 at 22:23

Canon experts: can the actual shutter count be determined from the EXIF?
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mattdmApr 2 '11 at 22:31

I just checked the EXIF and in all of my photos ImageNumber is 0 <aux:ImageNumber>0</aux:ImageNumber>
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HaiperlinkApr 2 '11 at 22:37

the factory does a test on all canon products so don't be surprised
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user6184Aug 3 '11 at 16:12

Most likely the shutter count is due to factory testing as noted in a previous answer. The fact that the number is 256 actually leads me to believe this in part because it a power of two (2^8 = 256) and is also the the maximum number that you can store in a byte of data (eight bits) so you tend to see it in use a lot by developers and the like.

As mentioned above, most DSLR shutters will work for 100,000s of shots, so even if the camera has been used for 3000 photos that is practically nothing. Most likely is that it's the lower number of 256, which would be about right for random product testing during manufacture.

Some cameras do come with a fairly accurate shutter count utility, but these are generally re-set if the camera has any shutter servicing or if the firmware has to be flashed.

There is no way to correctly retrieve the shutter count. There are some utilities for this (as mentioned above), which will surely give you a different and more accurate reading than what you derive from your file names, but even these utilities are not waterproof since the internal shutter count can always be reset by a technician.

My conclusion, the cam was already in use. Maybe it was used with an old SD-Card, so it started to count at 3500 but there were a minimum of 256 photos taken.
I'll bring the DSLR back to the store on monday.